It is well known that active noise cancellation is achieved by the electronic synthesis of an antinoise signal having the same frequency and amplitude as the noise, but 180 degrees out of phase, and then using the synthetic signal to generate and transmit an acoustic wave that mixes with the noise, so that the sum of the two waveforms approaches zero, thereby generating silence.
The basic components of an antinoise system include a microphone or transducer to monitor the noise source, and a signal processor to receive the noise signal and generate an antinoise signal having a mirror-image waveform. The antinoise signal is then passed to one or more speakers or transducers for generating the acoustic wave used to cancel the noise. In addition there is a synchronizing input which generates and feeds a correction signal to the processor, and an adaptor for constantly changing the cancellation signal to match a constantly changing noise signal.
The entire history of active sound cancellation has been directed to noise control, and to the filtering of unwanted sounds from speech transmission. For example, the radio receiver headset of an airline pilot has been equipped with means for cancelling background noise, so that the pilot hears noise-free voice reception. Such a device is described in U.S. Pat. No. 4,654,871. The patent also describes the addition of a noise cancellation signal to a transmitted voice signal that includes background noise, with the result that a noise-free signal is transmitted.